Method And System For Optical Measurements Of Contained Liquids Having A Free Surface

ABSTRACT

The present invention is an optical measurement system for measuring a liquid sample within a well. The system comprises a light source configured to transmit light though the well, a detector configured to measure optical signals derived from the transmitted light, and a tunable optical element. The tunable optical element is positioned between the light source and the well. The tunable optical element is operable to shape the light to compensate for distortions induced by a surface of the liquid sample. The detector is preferably located below the well for receiving a forward scatter signal indicative of at least one characteristic of the particles within the liquid sample.

RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationSer. No. 61/912,753, filed Dec. 6, 2013, titled “Method and System forOptical Measurements of Contained Liquids with the Free Surfaces,” whichis herein incorporated by reference in entirety.

COPYRIGHT

A portion of the disclosure of this patent document may contain materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patentdisclosure, as it appears in the Patent and Trademark Office patentfiles or records, but otherwise reserves all copyright rightswhatsoever.

FIELD OF THE INVENTION

The present invention relates generally to the field of opticalmeasurements of contained liquid samples. Specifically, the presentinvention relates to a system having a tunable optical element forobtaining high-sensitivity optical measurements from a liquid having afree surface that may have distortions, such as a meniscus. Such asystem may perform multiple optical measurements on samples contained ina two dimensional array of wells within a microplate.

BACKGROUND OF THE INVENTION

Many applications in the field of analytical research and clinicaltesting utilize optical methods for analyzing liquid samples. Amongthose methods are absorbance, turbidity, fluorescence/luminescence, andoptical scattering measurements. Optical laser scattering is one of themost sensitive methods, but its implementation can be very challenging,especially when analyzing biological samples in which suspendedparticles are relatively transparent in the medium. In this case, mostof the scattering process occurs in the forward direction near theincident laser beam. To detect this low-angle, forward scatteringsignal, high extinction of the incident beam is required. But variousoptical effects (e.g., such as laser beam spatial purity, opticalsurface scattering, and beam distortions by the free liquid surface)often interfere with the extinction of the incident beam. For thisreason, the forward scattering method is rarely applied in spite of itssensitivity.

In the case of fluorescence/luminescence detection, there is a spectralseparation between the excitation light and the emitted light, which canhelp to facilitate the extinction of the excitation light by means ofspectroscopic techniques, such as notches, bandpass optical filters, ormonochromators. But in many application cases, the fluorescence signalis many orders of magnitude lower compared to the excitation lightintensity and the excitation-light extinction by wavelength separationis not sufficient. For this reason, many systems collect the emittedlight from a direction that is opposite of or normal to the excitationbeam, such that the excitation light does not reach the detector.However, this can results in a rather complex optical layout, sometimesutilizing multiple detectors.

One particularly important application of optical measurements of liquidsamples involves a microplate reader used for microbiologic assays. Amicroplate comprises of multiple open-top wells containing individualsamples arranged in a two dimensional array (e.g., 8×12). To obtainuseful information on the samples content, the microplate reader mayutilize one or more types of optical measurements. Because of a twodimensional arrangement, the optical access is typically available inonly the top and bottom directions of the wells. The upper free surfaceof the liquid sample is normally curved due to the liquid's surfacetension. This curvature combined with the relatively small diameter ofthe wells cause a significant incident beam divergence or distortion,making its extinction very difficult and inefficient before it reachesthe detector. This is one reason that forward scatter signal measurementor a fluorescence signal measurement in the input-beam direction is noteasily implemented in wells of a microplate.

Accordingly, there is a need for an improved optical measurement systemthat allows for the detection of the forward scatter signals and/orforward fluorescence signals in the input-beam direction so as to allowfor a determination of the size, quantity, and/or concentration ofparticles (e.g., bacteria) in the liquid.

SUMMARY OF THE INVENTION

In one aspect, the present invention involves a method for overcomingthe aforementioned problems by generating an input beam with acontrolled distortion, and correcting the distortion induced by afree-liquid surface. As such, the method includes a substantiallycollimated or focused beam at the detector, while separating the desiredscattering signals and/or fluorescence signals by efficiently blockingthe input beam from detection.

In another aspect, the present invention involves an apparatus or systemequipped with a single detector array, for high sensitivity scatteringmeasurements at several scattering angles. The detector may receive backscatter signals, low-angle forward scatter signals, fluorescencesignals, and absorbance measurement signals, all using a single detectorarray

In yet a further aspect, the present invention is an optical measurementsystem for measuring a liquid sample. The system comprises a lightsource configured to transmit light though the well, a detectorconfigured to measure optical signals derived from the transmittedlight, and a tunable optical element. The tunable optical element ispositioned between the light source and the well. The tunable opticalelement is operable to shape the light to compensate for distortionsinduced by a surface of the liquid sample.

The present invention is also a method of determining at least onecharacteristic of particles within a liquid that is contained in a well.The method includes transmitting a light beam toward a free surface ofthe liquid and, prior to the light beam impinging on the free surface,altering a shape of the light beam to result in a substantiallycollimated or focused beam within the liquid after the input beam hasbeen subjected to optical distortions at the free surface. The methodfurther includes attenuating the input beam at a location outside of thewell and before a detector, and receiving forward scatter signals at thedetector. The forward scatter signals are indicative of at least onecharacteristic of the particles.

In yet a further embodiment, the present invention is an opticalmeasurement system for measuring at least one characteristic ofparticles within a liquid that is contained in a well. The systemcomprises a light source, a tunable optical element, a detector, and aninput-beam attenuator. The light source is for transmitting an inputbeam toward a free surface(s) of the liquid and in a manner that isgenerally parallel to a central axis of the well. The tunable opticalelement is located between the light source and the free surface(s) ofthe liquid. The tunable optical element is for altering the shape of theinput beam to compensate for a distortion associated with the freesurface(s) of the liquid. The detector is located below the well forreceiving a forward scatter signal indicative of at least onecharacteristic of the particles within the liquid. The input-beamattenuator is for inhibiting a transmitted portion of the input beamfrom impinging upon the detector. The transmitted portion of the inputbeam is the portion of the input beam that has transmitted through theliquid.

Additional aspects of the invention will be apparent to those ofordinary skill in the art in view of the detailed description of variousembodiments, which is made with reference to the drawings, a briefdescription of which is provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the various types of opticalmeasurements, each of which is individually known to be in the priorart, that may result from an input beam passing through a liquid in adirection transverse to the container's primary axis.

FIG. 2A is a perspective view of a microplate containing a plurality ofwells that containing liquid samples.

FIG. 2B is a schematic illustration of a system for taking an opticalmeasurement of a liquid along the primary axis of one of the wells ofthe microplate in FIG. 2A.

FIG. 3 is a schematic view of the optical measurement system with atunable optical element in accordance with the present invention.

FIG. 4A illustrates one method for adjusting the tunable optical elementin the system of FIG. 3.

FIG. 4B illustrates another method for adjusting the tunable opticalelement in the system of FIG. 3.

FIG. 5A illustrates one type of tunable optical element involving amechanically adjustable optical element.

FIG. 5B illustrates a second type of tunable optical element involvingan electronically or mechanically tunable lens.

FIG. 5C illustrates a third type of tunable optical element involving aspatial light modulator.

FIG. 6 is a schematic view of an alternative optical measurement systemthat takes multiple optical measurements on a single detector.

FIG. 7 illustrates an exemplary single detector for the system of FIG. 6that can detect multiple types of signals.

FIG. 8A illustrates a beam attenuation device in which only a centralportion of the input beam is attenuated, so as to allow an outer portionof the beam to be measured for absorbance of the liquid sample.

FIG. 8B illustrates the single detector of FIG. 7, whereby an absorbancemeasurement is also detected by use of the beam attenuation device ofFIG. 8A.

While the invention is susceptible to various modifications andalternative forms, specific embodiments will be shown by way of examplein the drawings and will be described in detail herein. It should beunderstood, however, that the invention is not intended to be limited tothe particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The drawings will herein be described in detail with the understandingthat the present disclosure is to be considered as an exemplification ofthe principles of the invention and is not intended to limit the broadaspect of the invention to the embodiments illustrated. For purposes ofthe present detailed description, the singular includes the plural andvice versa (unless specifically disclaimed); the words “and” and “or”shall be both conjunctive and disjunctive; the word “all” means “any andall”; the word “any” means “any and all”; and the word “including” means“including without limitation.”

FIG. 1 schematically illustrates the various optical measurements thatmay be taken on a vial 10 containing a liquid 12 having suspendedparticles (e.g., bacteria). The incident input beam 14 (also called an“excitation” beam in the case of fluorescence/luminescence) passesthrough the liquid 12 and interacts with the suspended particles. As oneexample, the input beam 14 may be derived from an LED.

The various types of optical measurements in FIG. 1 are individuallyknown in the prior art. A first detector 16 is located on the oppositeside of the input beam 14. The input beam 14 is partially absorbedwithin the liquid 12 such that an output beam 20 is received by thefirst detector 16 and is indicative of the absorbance of theparticle-filled liquid 12. Additionally, the receiver 16 can detect ascatter signal 22 resulting from the input beam 14 scattering from thevarious particles within the liquid 12. Additionally, a second receiver24 measures a high-angle side scatter signal 26 in a direction that isgenerally perpendicular to the input beam 14. And a third receiver 30measures the back scatter signal 32 caused by energy reflecting from theparticles in a direction generally opposite to the input beam 14.Finally, a portion of the input beam 14 is absorbed by particles, whichis then re-emitted at a wavelength that is characteristic to theparticle molecular structure. The process is known as fluorescence,which is a type of photo-luminescence. This fluorescence signal 40 isgenerally emitted in all directions, and can be picked up by all thedetectors sensitive in the spectral range of the emitted radiation. Itshould be noted that FIG. 1 is for the purpose of illustrating theindividual types of signals that are detectable when an input beam ispassed through a liquid. While these types of signal detectors areavailable individually within the prior art, all of their combinationsare not, however, necessarily a part of the state of the prior art.

In summary, there are several types of optical measurements availablewhen the input beam 14 is transmitted into the liquid 12 containingparticles. The first type of optical measurement detects thefluorescence signals 40, which is generally emitted in all directionsand can be emitted at multiple wavelengths. The second type of opticalmeasurement detects absorbance, which is performed by evaluating theintensity of the output light beam 20 that has passed through the liquid12. The third type of optical measurement involves light that iselastically scattered by the particles in the liquid 12 in variousdirections. As such, the input beam can be both absorbed and scatteredby the suspended particles, both of which prevent the incident inputbeam from reaching the first detector 16. The amount of absorbed andscattered light is related to the characteristics of the particles, suchas particle concentration and can be determined through known specialcalibration techniques. When the particles are bacteria, they can bedetected and counted by various techniques, which are generallydescribed in U.S. Pat. Nos. 7,961,311 and 8,339,601, both of which arecommonly owned and are herein incorporated by reference in theirentireties.

Regarding this third type of optical measurement involving scattering,there are three major types of scattering signals—back scatter signals32, forward scatter signals 22, and side scatter signals 26. Detectionof side scatter signals 26 is also known as nephelometry, whichevaluates a parameter (sometimes called “turbidity”) that, in certaincases, can be also be linked directly by calibration to particleconcentration.

FIG. 2A illustrates a common microplate 50 having a two-dimensionalarray of wells 52 extending downwardly from a top surface 54. Each ofthe wells 52 typically has an elongated shape and contains a liquidsample for measurement. However, optical access for all of the wells 52is possible only from the top and the bottom of the microplate 50, whileoptical access from the side is very limited for the interior wells 52.

FIG. 2B illustrates a typical layout for optical measurements detectedfrom a liquid 62 within one of the wells 52 of the microplate 50. Theliquid 62 has a free surface 64 that is characterized by a meniscus dueto the surface tension of the liquid 62. A laser 70 provides an inputbeam 74 that enters the liquid 62 from above the top surface 54 of themicroplate and is detected from the bottom. In particular, the inputbeam 74 results in an output beam 76, a forward scatter signal 78, and afluorescence signal 80 that are transmitted towards a detector 82.

If the fluorescence signal 80 is to be measured, an optical filter or amonochromator 84 is utilized to remove the incident output beam 76before it reaches the detector 82. But for either an optical filter or amonochromator 84, the ability to discriminate the wavelength of theincident output beam 76 from the fluorescence signal 80 is limited bythe filter's rejection ability or the monochromator's quality, which istypically on the order 10⁻⁵ to 10⁻⁸, respectively. But in many cases,the fluorescence signal 80 is weak, or the particle concentration in theliquid 62 is low, thereby requiring extremely low fluorescence signals80 to be measured. And for those cases, a higher rejection ratio shouldbe implemented, such that physical blocking of the incident output beam76 is required, which is problematic because it limits the ability toalso detect the fluorescence signal 80 from the bottom. As will bedescribed in more detail, the present invention resolves this problemwith the fluorescence signal 80 because the incident beam is blocked,leaving the fluorescence signal 80 (perhaps filtered through a filter)as the primary signal received at the detector.

The liquid 62 contained in the microplate well 54 typically has a highlycurved free surface 64 due to surface tension. This free surface 64 hasan optical power (sometimes with a significant optical aberration),substantially distorting the input beam 74 and causing it to diverge asshown in FIG. 2B. Because of this effect on the input beam 74, blockingthe output beam 76 or detecting the front-scatter signal 78 at lowangles is impossible. Accordingly, the only practical opticalmeasurements that are left in place are detection of the absorbance, theback scatter signal, and backward-emitted fluorescence. If absorbance ismeasured, there is no need for the optical filter or a monochromator 84in FIG. 2B. In summary, the divergence of the input beam 74 caused bythe liquid's free surface 64 negates the ability to effectively shieldor filter the detector 82 from the output beam 76, thereby inhibitingthe detection of the forward-emitted fluorescence 80 or the forwardscatter signal 78, leaving only absorbance to be detected from thebottom of the microplate 50. The present invention, which is generallydescribed in FIG. 3, helps to resolve this problem by correcting thedistortions caused by the free surface 64.

FIG. 3 illustrates one embodiment of the present invention that permitsthe detection of the forward scatter signal as well as the forwardemitted fluorescence. Specifically, the system includes a laser 110 asan optical source that produces an input beam 112 generally parallel tothe central axis of the well 52. In one preferred embodiment, the inputbeam 112 of the laser 110 is in a wavelength in the visible to nearinfrared (e.g., 300 nm to 2500 nm) and has a power in the range fromabout 0.10 milliwatts to 10 milliwatts. The input beam 112 is shaped bya tunable output element 120 located above the top surface 54 of themicroplate 50. The tunable output element 120 creates a specificallyshaped converging input beam 122 that enters the liquid 62 from the freesurface 64. Notably, the tunable output element 120 achieves a certainconvergence that negates the distorting effect caused by the meniscus atthe free surface 64, thereby resulting in a substantially collimatedbeam 124 (or a focused beam) that travels through the liquid 62 withinthe well 52 of the microplate 50 toward a detector 130. In oneparticular embodiment, the tunable output element 120 can be adjusted inthe range from −10 and +30 dpt for a well 52 with a 6.7 mm diameter. Thecollimated beam 124 is acted upon by a beam dump 132 (or other type ofattenuator) after leaving the well 154, but before reaching the detector130. In this case, the collimated beam 124 can be physically blocked bythe beam dump 132, enabling measurement of the low-angle forward scattersignal 140 and/or the fluorescence signal 142 with a high extinctionratio relative to the collimated beam 124. As such, the limitedgeometric area of the collimated beam 124 can be controlled, such thatthe detector 130 receives substantially only the forward scatter signal140 and/or the fluorescence signal 142.

When implementing an optical measurement system according to theschematic illustration of FIG. 3, it may be useful to include additionaloptical components. For example, a pinhole aperture (e.g., 0.5 mmpinhole) can be placed adjacent to the laser 110 to limit the outputbeam 112 to a specific size and shape. Similarly, the pinhole aperture(e.g., 0.1 mm to 1.0 mm pinhole) can also be placed below the tunableoutput element 120 and above the upper surface 54 the microplate 150 tohelp limit the light energy passed into the free surface 64 of theliquid 62.

FIGS. 4A and 4B illustrate feedback loops for the tunable output element120 illustrated in FIG. 3. In FIG. 4A, the detector 130 acquires thesignal from the collimated beam 124 after being acted upon (likelypartially) by the beam dump 132. The characteristics of the opticalsignal from the collimated beam 124 are received by a controller 148that executes an image acquisition algorithm at step 150 to extract datacorresponding to the position, the dimension, and the intensity of thecollimated beam 124 that are received on the image plane of the detector130. These optical signals received by the detector 130 can beconsidered as stray light. Based on the known characteristics of thefocused convergent beam 122 before entering the free surface 64 of theliquid 62, and the data corresponding to the position, dimension, andintensity distribution on the detector 130, a physical profile of thefree surface 64 is calculated or approximated. In other words, theoptical characteristics of the free surface 64 are now known, as if itwere a lens. The controller 148 executes a stray-light minimizationalgorithm at step 152 that calculates the wavefront compensation to beintroduced into the input beam 112 by the tunable optical element 120 tominimize the beam distortion caused by the free surface 64 of the liquid62, thereby achieving the collimated beam 124. The controller 148 thenadjusts the tunable optical element 120 at step 154, thereby alteringthe optical settings of the tunable optical element 120. The process maycontinue periodically, or iteratively, until the desired collimated beam124 is produced, thereby creating a minimum amount of stray light on thedetector 130. When achieved, the system can operate at a more optimumstate for receiving the forward scatter signal 140 and/or thefluorescence signal 142, as shown in FIG. 3.

In the feedback loop of FIG. 4B, the detector 130 is not used as part ofthe feedback loop. Rather, the free surface 64 of the liquid is measureddirectly or indirectly in other ways, such as a mechanical measurement,optical power measurement, or calculations based on knowing surfacetension parameters. The determined profile of the free surface 64 isused by the controller 148 to determine the optimal settings for thetunable optical element 120. Accordingly, the controller 148 receivesdata at step 160 to determine the surface profile of the free surface 64(e.g., measures reflections from the free surface 64). The controller148 executes a stray-light minimization algorithm at step 162 thatcalculates the wavefront compensation to be introduced into the inputbeam 112 by the tunable optical element 120 to minimize the beamdistortion caused by the free surface 64 of the liquid 62 and achievethe collimated beam 124. The controller 148 then adjusts the tunableoptical element 120 at step 164 to alter the optical settings of thetunable optical element 120. The process continues periodically, oriteratively, until identifying the desired convergent beam 122 thatexits the tunable optical element 120 will result in the desiredcollimated beam 124.

FIGS. 5A-5C illustrate exemplary implementations of the tunable opticalelement 120. In FIG. 5A, the tunable optical element 120 a is amechanically adjustable assembly comprising a first structure 180attached to first optical element 182 (such as a lens) and a secondstructure 182 attached to a second optical element 184. In response tocontrol signals received from the controller 148, an actuator 188 (suchas a motor) imparts a force on the first structure 180 and/or the secondstructure 184 to adjust the distances between the first optical element182 and the second optical element 184. The adjustment of the distancesbetween the first optical element 182 and the second optical element 184affects the shape of the convergent beam 122 (FIG. 3) that exits thetunable optical element 120 a.

In FIG. 5B, the tunable optical element 120 b includes a mountingstructure 190 that holds a mechanically or electronically tunable lens192. The controller 148 adjusts the electrical output of a circuit 194that directly affects the shape of the electronically tunable lens 192.Alternatively, the controller 148 adjusts the output of the circuit 194to alter the shape of the mechanical mounting structure 190, therebyindirectly affecting the shape of the tunable lens 192. The adjustmentof the shape of the tunable lens 192 affects the shape of the convergentbeam 122 (FIG. 3) that exits the tunable optical element 120 b.

In FIG. 5C, the tunable optical element 120 c includes a spatial lightmodulator (SLM) 200. The controller 148 adjusts the SLM 200 to provide agreat amount of control of the beam 122 that transmits through the freesurface 64. The SLM 200 may compensate for virtually any surfacedistortion free surface 64, but at a limited optical power.

The tunable optical element 120 may also be a combination of any of theabove elements in FIGS. 5A-5C such as, for example, a tunable lens withan SLM. Mechanically controlled telescopes, deformable adaptive mirrors,or other arrays of tunable optical elements can also be utilized for thefunctionality required by the tunable optical element 120.

FIG. 6 illustrates an alternative system that can receive multipleoptical measurements when an input beam is transmitted along the primaryaxis of a vial or well. Further, the system receives the multipleoptical measurements on a single detector 230. The exemplary system inFIG. 6 is illustrative for implementation of various components used formultiple optical measurements, which can be combined in any otherarrangements

In FIG. 6, a back scatter signal 240 is collected by an optical fiber242 near the free surface 64 of the liquid 62. The collection of theback scatter signal 240 may be facilitated by the addition of opticalelements in front of the optical fiber 242 or by shaping the end facetof the optical fiber 242. The back scatter output signal 244 from theend of the fiber 242 is located at a physical position on the detector230 that is unused for the measurement of the forward scatter signal 140or the fluorescence signal 142.

In cases when optical access generally normal to the input beamdirection is possible, a side scatter signal 250 and/or a sidefluorescence signal 251 may be transmitted by a second optical fiber 252to the detector 230. The optical fiber 252 is located near the vial'sside surface and may collect the side scatter signal 250 and/or the sidefluorescence signal 251 with additional coupling optics (as shown inFIG. 6), although additional coupling optics are not necessary. Theoutput light 256 exiting the output end of the second optical fiber 252is coupled to the detector 230 using a prism 254. Because the detector230 must be located near the bottom surface of the well, the prism 254allows the second optical fiber 252 to enter the detector region fromthe horizontal direction, thereby minimizing the height requirement formechanical access to the detector 230. One or both sides of the prism254 may contain a dielectric coating for rejection of the incident lightwavelength, which can be useful in fluorescence measurements. The outputsignal(s) 256 from the side scatter signal 250 and the side fluorescencesignal 251 can also be spatially separated via optics (like the prism254) before being transmitted to the detector 230. The output signal(s)256 from the side scatter signal 250 and/or a side fluorescence signal251 are then sent to a second physical position(s) on the detector 230not used for the forward scatter signal 140 or the fluorescence signal142, which are transmitted past the beam dump 132. Accordingly, whenside access is available to a vial, several types of opticalmeasurements of the liquid 62 can be detected by the single detector 230and the information can then be used to determine the types, amounts,and/or concentrations of the particles located within the liquid 62.

FIG. 7 illustrates an exemplary layout of the detector 230 for multiplemeasurements from a liquid sample. The dashed circular line region 324of the detector 230 represents a physical position of the blocked beamdue to the beam dump 132. The forward scatter signal 140 is detected ina region 340 of the detector 230 that is located concentrically aroundthe dashed circular line region 324. The various optical fibers (e.g.,the optical fibers 242, 252 in FIG. 6) from the top and sides of thevial carry several complementary signals that are also detected by thesame detector 230. For example, the back scatter signal 244 from thefirst optical fiber 242 can be detected at the top middle region 344 ofthe detector 230. The side scatter signal 250 can be measured at theleft middle region 350. A first fluorescence signal can be detected atthe lower middle region 351 a (e.g., within a first wavelength band orat a different angle), while a second fluorescence signal can bedetected at the lower left region 351 b (e.g., within a secondwavelength band or at a different angle) of the detector 230. Anadditional side scatter signal at a different angle can also be detectedat the top left region 360 of the detector 230. Of course, more or lessoutput signals can be detected by the detector 230. Furthermore, whileFIG. 7 is provided for the purpose of illustrating multiple measurementson a single detector, only a single measurement (forward scatter signal140 in FIG. 3) can be measured, as would be the case for the detector130 when no forward fluorescence is to be detected.

FIG. 8A illustrates a beam attenuation device 380 that can be usedinstead of the beam dump 132 (FIGS. 3, 4, and 6). In the beamattenuation device 380, a central portion 382 of the incident beam thathas traveled though the liquid 62 is attenuated, and the peripheralportion 384 is blocked, preferably by an opaque structure.

In FIG. 8B, the beam attenuation device 380 allows for an absorbancemeasurement in the right middle region 392 of the detector 230. Theperipheral region 394 on the detector 230 around the right middle region392 has substantially no signal due to the preferably opaque peripheralportion 384 of the beam attenuation device 380. Because the forwardscatter signal 140 (FIG. 6) is particularly useful at low particleconcentrations or with weak scattering signals, simultaneous absorbanceand scattering measurements are challenging because a wide dynamic rangeof the detector 230 is required. According to this aspect of the presentinvention, a part of the input beam is partially transmitted to thecentral region 392 of the detector 230 via the beam attenuation device380, bringing it to intensities low enough that the same detector 230can measure it simultaneously with the forward scatter signal 140. Usingthis alternative arrangement of FIGS. 8A and 8B, an absorbancemeasurement can also be carried out, enabling measurement of highparticle concentrations. Furthermore, the present invention contemplatesa system that uses both the beam dump 132 and the beam attenuationdevice 380 that are separately moved into and out of the path of theincident beam to allow the same detector 230 to provide the outputdetection patterns illustrated in both FIG. 7 and FIG. 8B.

Because it is advantageous to limit the number of back reflections thatare detected by the detector 130, 230, the present invention alsocontemplates the use of an angled surface at the bottom of each well (asopposed to a horizontal surface) within the microplate, or the bottomsurface of an individual vial. The angled surface helps to eliminatesome of the back reflections that are incident upon the detector 130,230. Furthermore, it is also advantageous to have the sidewalls of eachof the wells be made of absorbing material (such as a black plastic) toabsorb some of the retro-reflections. The bottom surface is preferablyvery thin, transmissive, and has a surface roughness below 10 nm.

It should be noted that the present invention has been describedrelative to a free surface of a liquid located within aliquid-containing well. However, the present invention is useful on oneor more free surfaces, such as the free surface of a drop of a liquidsample that has a curved free surface or surfaces. Furthermore, whilethe sample-containing well has been described as being elongated, thewell could also have a much more shallow shape. For example, the lengthof the well may have a dimension that is similar to its diameter.

Each of these embodiments and obvious variations thereof is contemplatedas falling within the spirit and scope of the claimed invention, whichis set forth in the following claims. Moreover, the present conceptsexpressly include any and all combinations and subcombinations of thepreceeding elements and aspects.

1. (canceled) 2-20. (canceled)
 21. An optical measurement system formeasuring at least one characteristic of particles within liquidsamples, comprising: a microplate having a plurality of wells positionedin a two-dimensional arrangement; a light source for transmitting aninput beam toward a free surface of a liquid sample in one of theplurality of wells and generally parallel to a central axis of the oneof the plurality of wells; a tunable optical element located between thelight source and the free surface of the liquid sample, wherein thetunable optical element is operable to alter the shape of the input beambetween a first shape resulting in the input beam not being collimatedand a second shape resulting in collimating the input beam to compensatefor a distortion associated with the free surface of the liquid sample;a detector located below the one of the plurality of wells for receivinga forward scatter signal indicative of at least one characteristic ofthe particles within the liquid sample in the one of the plurality ofwells; and an input-beam attenuator to inhibit a transmitted portion ofthe input beam from impinging upon the detector, the transmitted portionof the input beam being the portion of the input beam that hastransmitted through the liquid sample.
 22. The optical measurementsystem of claim 21, wherein the input-beam attenuator is a beam blocklocated adjacent to the detector and below a bottom surface of the oneof the plurality of wells.
 23. The optical measurement system of claim21, wherein the input beam causes a fluorescence signal to be emittedfrom the particles, the detector further receiving a fluorescence signalindicative of at least one characteristic of the particles within theliquid sample.
 24. The optical measurement system of claim 21, whereinthe particles include bacteria and the at least one characteristicindicated by the forward scatter signal is the amount of bacteria withinthe liquid sample.
 25. The optical measurement system of claim 21,further including at least one optical fiber to gather additionalsignals related to particle characteristics from a side or a top of theone of the plurality of wells, the optical fibers providing theadditional signals to the detector.
 26. The optical measurement systemof claim 21, further including a controller coupled to the tunableoptical element, the controller for tuning optical parameters associatedwith the tunable optical element so as to alter the shape of the inputbeam.
 27. The optical measurement system of claim 26, wherein thecontroller receives information from the detector for tuning the opticalparameters so as to properly compensate for the distortion associatedwith the free surface.
 28. The optical measurement system of claim 26,wherein the controller receives information associated with the freesurface of the liquid sample for tuning the optical parameters toproperly compensate for the distortion associated with the free surface.29. The optical measurement system of claim 21, wherein the tunableoptical element is at least one of a group consisting of a mechanicallyactuated adjustable lens, an electronically shaped lens, and a spatiallight modulator.
 30. The optical measurement system of claim 21, whereineach of the plurality of wells has an elongated shape having a top foroptical access of the input beam and a bottom.
 31. The opticalmeasurement system of claim 21, wherein each of the plurality of wellshas a shape having a diameter substantially the same as a length of theshape, and wherein the wells each have a top for optical access of theinput beam and a bottom.
 32. A method of determining at least onecharacteristic of particles within a liquid that is contained in a wellof a plurality of wells arranged in an array in a microplate,comprising: transmitting a light beam toward a free surface of theliquid in one of the plurality of wells; prior to the light beamimpinging on the free surface, altering a shape of the light beam via atunable optical element, between a first shape resulting in the inputbeam not being collimated and a second shape resulting in collimatingthe input beam to result in a substantially focused beam shape withinthe liquid in the well after the input beam has been subjected tooptical distortions at the free surface; attenuating the input beam at alocation before a detector; and receiving forward scatter signals at thedetector, the forward scatter signals being indicative of at least onecharacteristic of the particles.
 33. The method of claim 32, wherein theparticles include bacteria and the at least one characteristic indicatedby the forward scatter signal is the amount of bacteria within theliquid sample.
 34. The method of claim 32, further comprising gatheringadditional signals related to particle characteristics from a side or atop of the one of the plurality of wells via an optical fiber; andproviding the additional signals to the detector.
 35. The method ofclaim 32, further comprising tuning optical parameters associated withthe tunable optical element so as to alter the shape of the input beamvia a controller.
 36. The method of claim 35, wherein the controllerreceives information from the detector for tuning the optical parametersto properly compensate for the distortion associated with the freesurface.
 37. The method of claim 35, wherein the controller receivesinformation associated with the free surface of the liquid sample fortuning the optical parameters so as to properly compensate for thedistortion associated with the free surface.
 38. The method of claim 32,wherein the tunable optical element is at least one of a groupconsisting of a mechanically actuated adjustable lens, an electronicallyshaped lens, and a spatial light modulator.
 39. The method of claim 32,wherein each of the plurality of wells has an elongated shape having atop for optical access of the input beam and a bottom.
 40. The method ofclaim 32, wherein each of the plurality of wells has a shape having adiameter substantially the same as a length of the shape, and whereinthe wells each have a top for optical access of the input beam and abottom.